The present invention relates to data communications equipment, e.g., modems. In particular, this invention relates to the transmission of both voice and data signals over the same communications facility at the same time.
While voice signals; have a large dynamic amplitude range, they have an amplitude probability distribution heavily weighted toward zero, i.e., voice is mostly very small in amplitude with only occasional bursts to near full scale. To improve performance, telecommunications networks, e.g., the public switched telephone network (PSTN), employ companders to amplify weak voice signals in relation to the large voice signals. This improves the signal to noise ratio (SNR) by raising the average voice signal level above the noise floor of the communications network.
The co-pending, commonly assigned, U.S. Patent application of Gordon Bremer et al. entitled "Simultaneous Analog and Digital Communication," Ser. No. 08/076,505, filed on Jun. 14, 1993, discloses a simultaneous voice and data communication system in which voice signals are added to two-dimensional Quadrature Amplitude Modulated (QAM) data symbols for transmission over a communications channel, e.g., the PSTN, to a receiving modem. In particular, each data symbol is a reference signal point value to which is added a voice signal vector, which represents a voice signal. The addition of the voice signal vector to the data symbol results in a signal point being selected that is a function of both the selected data symbol and the size of the voice signal vector. Unfortunately, since the voice signal is effectively added to the data signal, the voice signal is biased with a different offset for each selected data symbol. The PSTN does not expect a voice signal with, what is effectively, a DC offset. Consequently, this results in low-level voice signals getting no gain from PSTN compandors--which, overall, degrades the quality of the voice transmission through the PSTN.